1. Field of the Invention
The present invention relates to a spin valve sensor having a pinned layer structure composed of cobalt iron vanadium (CoFeV) and, more particularly, to such a pinned layer structure that improves read signal symmetry of a read head.
2. Description of the Related Art
The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a spin valve sensor for sensing the magnetic signal fields from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive spacer layer sandwiched between a ferromagnetic pinning layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90xc2x0 to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the rotating disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic signal fields from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is preferably parallel to the ABS, is when the sense current is conducted through the sensor without magnetic field signals from the rotating magnetic disk. If the quiescent position of the magnetic moment is not parallel to the ABS the positive and negative responses of the free layer will not be equal which results in read signal asymmetry which is discussed in more detail hereinbelow.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layers are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with the pinned and free layers. When the magnetic moments of the pinned and free layers are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. An increase in scattering of conduction electrons increases the resistance of the spin valve sensor and a decrease in scattering of the conduction electrons decreases the resistance of the spin valve sensor. Changes in resistance of the spin valve sensor is a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layers. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals from the rotating magnetic disk. The sensitivity of the spin valve sensor is quantified as magnetoresistance or magnetoresistive coefficient dr/R where dr is the change in resistance of the spin valve sensor from minimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the spin valve sensor at minimum resistance. Because of the high magnetoresistance of a spin valve sensor it is sometimes referred to as a giant magnetoresistive (GMR) sensor.
The transfer curve for a spin valve sensor is defined by the aforementioned cos xcex8 where xcex8 is the angle between the directions of the magnetic moments of the free and pinned layers. In a spin valve sensor subjected to positive and negative magnetic signal fields from a moving magnetic disk, which are typically chosen to be equal in magnitude, it is desirable that positive and negative changes in the resistance of the spin valve read head above and below a bias point on the transfer curve of the sensor be equal so that the positive and negative readback signals are equal. When the direction of the magnetic moment of the free layer is substantially parallel to the ABS and the direction of the magnetic moment of the pinned layer is perpendicular to the ABS in a quiescent state (no signal from the magnetic disk) the positive and negative readback signals should be equal when sensing positive and negative fields that are equal from the magnetic disk. Accordingly, the bias point should be located midway between the top and bottom of the transfer curve. When the bias point is located below the midway point the spin valve sensor is negatively biased and has positive asymmetry and when the bias point is above the midway point the spin valve sensor is positively biased and has negative asymmetry. The designer strives to improve asymmetry of the readback signals as much as practical with the goal being symmetry. When the readback signals are asymmetrical, signal output and dynamic range of the sensor are reduced.
Readback asymmetry is defined as             V      1        -          V      2            max    ⁡          (                        V          1                ⁢                  xe2x80x83                ⁢        or        ⁢                  xe2x80x83                ⁢                  V          2                    )      
For example, +10% readback asymmetry means that the positive readback signal V1 is 10% greater than it should be to obtain readback symmetry. 10% readback asymmetry is acceptable in many applications. +10% readback asymmetry may not be acceptable in applications where the applied field magnetizes the free layer close to saturation. In these applications +10% readback asymmetry can saturate the free layer in the positive direction and will reduce the negative readback signal by 10%. An even more subtle problem is that readback asymmetry impacts the magnetic stability of the free layer. Magnetic instability of the free layer means that the applied signal has disturbed the arrangement or multiplied one or more magnetic domains of the free layer. This instability changes the magnetic properties of the free layer which, in turn, changes the readback signal. The magnetic instability of the free layer can be expressed as a percentage increase or decrease in instability of the free layer depending upon the percentage of the increase or decrease of the asymmetry of the readback signal. Standard deviation of the magnetic instability can be calculated from magnetic instability variations corresponding to multiple tests of the free layer at a given readback asymmetry. There is approximately a 0.2% decrease in standard deviation of the magnetic instability of the free layer for a 1% decrease in readback asymmetry. This relationship is substantially linear which will result in a 2.0% reduction in the standard deviation when the readback asymmetry is reduced from +10% to zero. The magnetic instability of the free layer is greater when the readback asymmetry is positive than when the readback asymmetry is negative.
The location of the transfer curve relative to the bias point is influenced by four major forces on the free layer of a spin valve sensor, namely a ferromagnetic coupling field HFC between the pinned layer and the free layer, a net demagnetizing (demag) field HD from the pinned layer, a sense current field HI from all conductive layers of the spin valve except the free layer and a net image current field HIM from the first and second shield layers. The sense current field HI is typically greater a sum of the other magnetic fields HFC, HD and HIM and is difficult to counterbalance to achieve readback signal symmetry.
When the sense current IS is applied to the spin valve sensor there is an image sense current in each of the first and second shield layers. The image sense current in each shield layer causes each shield layer to produce an image sense current field HIM which traverses the free layer in a direction that is substantially perpendicular to the ABS. When the free layer of the AP pinned spin valve is symmetrically located midway between the first and second shield layers the image sense current fields counterbalance each other so that the net image sense current field on the free layer is zero. When the free layer is located asymmetrically between the first and second shield layers, hereinafter referred to as gap offset, the aforementioned net image sense current field can be employed for counterbalancing the other magnetic fields on the free layer. This is accomplished by sizing the first and second gap layers that separate the free layer from the first and second shield layers respectively so that the free layer is closer to a selected one of the shield layers. With increasing linear densities of magnetic read heads, a gap offset becomes impractical because of the risk of shorting between first and second lead layers to the spin valve sensor and the shield layers. For instance, in a bottom spin valve, where the free layer structure is closer to the second shield layer than to the first shield layer, the second read gap is typically narrower than the first read gap so that the second shield layer exerts a net imaging current field HIM on the free layer structure for counterbalancing other fields acting thereon. If this second read gap gets too narrow the thickness of the second read gap layer (G2), which is composed of alumina, will be too thin to prevent the lead layers from shorting to the second shield layer. Since the total read gap is made narrower in order to promote higher linear density of the read head, it becomes difficult to make a gap offset without shorting the lead layers to the second shield layer. The opposite situation is true for a top spin valve where the free layer structure is closer to the first shield layer than to the second shield layer.
It is desirable to employ a metallic pinning layer with the preference being platinum manganese (PtMn). Platinum manganese (PtMn) has a high blocking temperature (375xc2x0 C.) which must occur before magnetic spins in the platinum manganese (PtMn) are free to rotate in response to an extraneous magnetic field. This provides the read head with high thermal stability. Unfortunately, a platinum manganese (PtMn) pinning layer causes an additional sense current field on the free layer. However, the platinum manganese (PtMn) pinning layer causes a negative ferromagnetic coupling field from the pinned layer structure on the free layer structure which is additive with the net demagnetizing field HD for counterbalancing the sense current field HI.
An improved spin valve sensor, which is referred to hereinafter as antiparallel pinned (AP) spin valve sensor, is described in commonly assigned U.S. Pat. No. 5,465,185 to Heim and Parkin which is incorporated by reference herein. The AP spin valve differs from a single pinned layer spin valve in that the AP pinned layer structure has an antiparallel coupling layer which is sandwiched between ferromagnetic first and second layers. The first AP layer, which may comprise several thin films, is immediately adjacent to the antiferromagnetic pinning layer and is exchange-coupled thereto, with its magnetic moment directed in a first direction. The second AP pinned layer is immediately adjacent to the free layer and is exchange-coupled to the first AP pinned layer by the minimal thickness (in the order of 6 xc3x85) of the antiparallel coupling layer between the first and second AP pinned layers. The magnetic moment of the second AP pinned layer is oriented in a second direction that is antiparallel to the direction of the magnetic moment of the first AP pinned layer. The magnetic moments of the first and second AP pinned layers subtractively combine to provide the AP pinned layer structure with a net magnetic moment. The direction of the net moment is determined by the thicker of the first and second AP pinned layers. The thicknesses of the first and second AP pinned layers are chosen so that the net moment is small. A small net moment equates to a small demagnetizing (demag) field exerted on the free layer by the AP pinned layer structure. Further, since the antiferromagnetic exchange coupling is inversely proportional to the net moment, this results in a large exchange coupling between the pinning and pinned layers.
Because of the strong exchange coupling between the AP pinned layer structure and the pinning layer, the AP pinned layer structure is preferred to a single pinned layer. A disadvantage of the AP pinned layer structure, as compared to the single pinned layer, is that the demag field from the AP pinned layer structure is smaller and therefore does not contribute as much in counterbalancing the large sense current field HI. Another disadvantage of the AP pinned layer structure is that it has more conductive material than the single pinned layer which increases the sense current field HI. The same is true for the preferred platinum manganese (PtMn) pinning layer which also adds conductive material which increases the sense current field HI. Accordingly, it would be desirable to provide a spin valve sensor with the preferred AP pinned layer structure and the preferred platinum manganese (PtMn) pinning layer and yet decrease the sense current field HI so as to promote readback signal symmetry. Further, it would be desirable to accomplish this result without implementing a gap offset which increases the risk of shorting of the lead layers to the second shield layer because of pin holes in the second read gap layer.
The present invention reduces the sense current field HI by providing a pinned layer structure which is composed of cobalt iron vanadium (CoFeV). The vanadium (V) increases the resistance of the pinned layer structure which means that less sense current will be conducted through the pinned layer structure. Since there is less sense current conducted through the pinned layer structure the pinned layer structure produces less sense current field. In a preferred embodiment the cobalt iron vanadium (CoFeV) pinned layer structure is made thinner than prior art pinned layer structures with its cobalt (Co) and iron (Fe) content adjusted to increase its magnetic moment as desired. It has been found that cobalt iron vanadium (Co49Fe49V2) has a resistance four times that of cobalt iron (Co90Fe10). As compared to cobalt iron (Co90Fe10), which is typically employed in spin valve sensors, the cobalt iron vanadium (Co49Fe49V2) has a higher magnetic moment because of the increased iron (Fe) content. Cobalt iron vanadium (Co49Fe49V2) has about 50% more moment than cobalt iron (Co90Fe10). With the preferred AP pinned layer structure the first AP pinned layer, which interfaces the pinning layer, may be composed of cobalt iron vanadium (Co49Fe49V2) while the second AP pinned layer, which interfaces the spacer layer, may be cobalt iron (Co90Fe10). The higher cobalt content in the second AP pinned layer next to the spacer layer has been found to increase the magnetoresistive coefficient dr/R.
In another embodiment both of the first and second AP pinned layers may be composed of cobalt iron vanadium (Co49Fe49V2). This embodiment would significantly reduce the sense current field produced by the AP pinned layer structure. In still a further and preferred embodiment the first AP pinned layer is composed of cobalt iron vanadium (Co49Fe49V2) and the second AP pinned layer has first and second films, wherein the first film is composed of cobalt iron vanadium (Co49Fe49V2) and the second film is composed of cobalt iron (Co90Fe10) with the second film interfacing the spacer layer. With this arrangement the resistance of the AP pinned layer structure can be optimized while retaining the desirable high cobalt (Co) content interface of the AP pinned layer structure with the spacer layer.
In a single pinned layer embodiment the single pinned layer may be composed of cobalt iron vanadium (Co49Fe49V2). In another embodiment of the single pinned layer the single pinned layer may comprise first and second films wherein the first film is cobalt iron vanadium (Co49Fe49V2) and the second film is cobalt iron (Co90Fe10). In this embodiment the second film interfaces the spacer layer so that the magnetoresistive coefficient dr/R can be increased while still optimizing a high resistance of the single pinned layer. Because of the high resistance of the pinned layer structure in each of the aforementioned embodiments there is less sense current bypassing the free layer structure which will improve the magnetoresistive coefficient dr/R of the sensor.
An object of the present invention is to provide a pinned layer structure which improves readback asymmetry and decreases sense current shunting.
Another object is to provide an AP pinned layer structure wherein a first AP pinned layer reduces a sense current field and sense current shunting and a second AP pinned layer optimizes a magnetoresistive coefficient dr/R of the read head.
A further object is to provide an AP pinned layer structure wherein each of first and second AP pinned layers reduces sense current shunting and a sense current field.
Still another object is to provide an AP pinned layer structure wherein the first AP pinned layer reduces sense current shunting and a sense current field and the second AP pinned layer reduces sense current shunting and a sense current field as well as optimizing a magnetoresistive coefficient dr/R.
Still a further object is to provide a single pinned layer structure which reduces sense current shunting and a sense current field as well as optimizing a magnetoresistive coefficient dr/R.
Other objects and advantages of the invention will become apparent upon reading the following description taken together with the accompanying drawings.